Providing feedback in a multiple-input multiple-output (MIMO) communication environment has proven to significantly benefit performance. In early systems that employed feedback, weighting factors were created at the MIMO receiver based on channel conditions. A codeword was fed back from the MIMO receiver to the transmitter, which would apply the weighting factors to the respective signals to be transmitted from the different MIMO transmitter antennas. The weighting factors effectively pre-distorted the signals to be transmitted to reverse the effects of the communication channel. Accordingly, the signals received at the MIMO receiver's antennas approximated those that would have been received without weighting through a clear channel. Unfortunately, channel conditions frequently if not continuously change, and the weighting factors to represent channel conditions are data-intensive. In light of the limited bandwidth available for feedback, providing weighting factors for changing channel conditions became unfeasible.
In an effort to reduce the bandwidth required to provide feedback for a MIMO channel, designers developed codebooks, which are maintained at the MIMO transmitter and MIMO receiver. The codebooks include codewords, which are predefined weighting factors. These predefined weighting factors are configured to cover the range of possible channel conditions through a fixed number of discrete weighting factors. Each of the codewords in a codebook is associated with a codeword index. In operation, the MIMO receiver will systematically determine the channel conditions and select the most appropriate codeword in light of the channel conditions. Instead of sending the codeword, which includes the weighting factors, the MIMO receiver will send the codeword index to the MIMO transmitter over an appropriate feedback channel. The MIMO transmitter will receive the codeword index, obtain the weighting factors of the corresponding codeword, and apply those weighting factors to the signals to be transmitted from the different transmit antennas.
The number of codewords in the codebook impacts the bandwidth required for feedback and the performance enhancement associated with using feedback. Unfortunately, as the number of codewords increases, the bandwidth required to feed back the codeword index to the MIMO transmitter from the MIMO receiver increases. Although incorporating the use of codebooks and providing the codeword index instead of the entire codeword as feedback has provided performance enhancement over systems providing the entire codeword as feedback, there is still a need to increase the number of available codewords while minimizing the bandwidth required to provide feedback from the MIMO receiver to the MIMO transmitter.